Vehicle systems and components are tested during the development and manufacturing thereof using various techniques. The systems and components undergo static, quasi-static, and dynamic testing to meet and exceed various durability, lifespan, and performance requirements. The testing techniques are used to simulate a working environment and to assure that the systems and/or components under test can survive and perform as desired within that environment.
In performing such tests various actuators are utilized. The actuators apply force on the systems and or components under test. It is desirable that the applied force be accurately controlled. It is also desirable that the actuators when appropriate be capable of following the motion of a device without imposing uncharacteristical forces or in effect restricting the motion of that device. For example, when testing an airfoil or wing of an aircraft tens to hundreds of actuators may be coupled to both sides of that wing. The actuators must work in unison to flex the structure and cause accelerated fatigue. Some tests require high cycle rates, which can be difficult to follow due to changing dynamics experienced by the actuators. As an example, when the wing is being flexed in an upward direction, the actuators coupled to the topside of that wing ought to follow the motion and velocity of that wing without uncharacteristically restricting its movement while the bottom side actuators are applying force in an upward direction. In other words, the non-acting or force-imposing actuators should not impose forces on the wing that would not normally be experienced in a normal operating environment.
Dynamic load control involves the accurate application of compressive and tensile forces to a moving object. When this load is applied using hydraulic actuators and servo valves with classical control methods, the resulting accuracy may be unacceptable at maximum rated pressures and flows. There may be inaccuracy in the applied force relative to the commanded force signals due to motion of the object. This inaccuracy is affected to such a degree that the motion of the object may be drastically reduced to obtain the desired force control accuracy.
Force control systems can also experience lack of symmetry in system polarity. This is especially true when utilizing single ended actuators, in which the amount of lack of symmetry is significant. The lack of symmetry can be due to differences in tubing connections, servo valves, and other components and devices.
Many force control systems, such as lag-lead type, lead-lag type, and pole-zero placement type systems, tend to increase system loop gain in order to reduce steady state error and thus compensate for the motion of the tested object. However, the increased gain may only marginally reduce the error and leads to limit cycling or oscillation and instability in the force control loop. Even with the increased gain, these types of force control systems may not be robust when null shifts occur in the equipment. Null shifts may cause a proportionate amount of force errors to accumulate. Thus, these systems are partially uncontrolled systems and may only be marginally stable and as error increases the systems may become unstable and may be sensitive to null shifts in the hardware.
Another method used to increase system robustness and reduce steady state error is error integration, which is successful in slower systems that do not have backlash and other non-linearities due to mechanical linkages. Backlash can be introduced from, for example, pin-slop. Although useful for static conditions, error integration can cause instability during transitions in force polarity. The integration process increases the settling time required for the actuator to apply an accurate load. This added time constraint reduces the benefit of using integral compensation on durability tests that have fast changing set point commands.
Yet another method used to reduce steady state error is referred to as damping derived from the force signal. This method tends to be slow, which results in larger errors for longer time intervals. The errors are larger due to the increased time for the damping algorithm to correct for the velocity.
Multiple force control systems that incorporate velocity and acceleration compensation are provided in the U.S. patent application Ser. No. 11/221,006, filed Sep. 7, 2005, entitled “VELOCITY FEEDBACK COMPENSATION FOR FORCE CONTROL SYSTEMS” (hereinafter the '006 patent application), which is incorporated by reference herein. These systems overcome steady state errors, increased settling time, and other associated disadvantages commonly associated with force control systems.
However, another common associated disadvantage of force control systems may be an inability to account for force transients. Force transients refer to resultant forces that may be experienced due to object velocity reversals or, in other words, resultant forces due to change in travel direction of an object. Force transients can also occur due to vibrations, shocks, or large abrupt random forces exerted on the system. Although the force control systems of the '006 patent application provide some reduction in force transients, additional reduction is desired.
Several methods have been used to increase stability in force control systems. One method is to reduce the proportional loop gain. This unfortunately may tend to reduce the accuracy, increase errors, and reduce bandwidth or system response. Other methods have included increasing the response of a servo control system, using higher response actuators, using anticipatory logic, lead-lag compensation, lag-lead compensation, notch filtering, pole- zero compensation, and gain scheduling. The stated methods have resulted in marginal stability, reduced stability, stability over limited frequency bandwidth, limits in force output, large, heavy, and expensive actuators, and/or an increase in phase lag at certain frequencies. Increasing the response of a servo control system may be ineffective because the transient disturbance rate is high. Some force control systems as designed are incapable of being easily altered to provide improved stability. In addition, the motion that causes the force transients may be activated by a mechanism that is quicker than the associated force control system, thus preventing the system from being able to account for such transients.
Thus, there exists a need for an improved force control system that accounts for and minimizes the generation of force transients and improves stability of the control system.